The invention relates to a differential pressure sensor and to a method of fitting it. More precisely, the invention concerns a differential pressure sensor with an isostatically mounted pressure measuring cell.
Differential pressure cells are used in particular for measuring small pressure differences between great static pressures. Therefore, the cells have to be dimensioned to withstand the static pressures. In the case of a measuring cell comprising two half-cells which are separated from each other by means of a central diaphragm, particularly the joints between the separating diaphragm and the half-cells must therefore also be of a correspondingly strong design, in order for example to withstand the strong peel stresses which occur on account of the great difference in pressure between the inside of the cell and the surroundings. However this inevitably leads to a stiffening of the measuring diaphragm or similar impairments, which ultimately impair the measuring accuracy of the differential pressure sensor.
For the purpose of improved measuring accuracy, it is desirable to relieve the joints and consequently reduce the dimensioning requirements. In this respect, Bonne discloses in U.S. Pat. No. 5,684,253 a differential pressure sensor in which the pressure measuring cell is arranged in a chamber of a housing, with the pressure supply line to one half-cell of the pressure measuring cell having a branch, so that the medium from the pressure supply line also surrounds the measuring cell and subjects it to the measuring pressure in question. In this way, the pressure difference between the interior of the half-cell and the surroundings of the pressure measuring cell is largely eliminated. If the pressure in the second half-cell is at a similar level to that in the first half-cell, the arrangement described offers adequate protection for constant pressure conditions. However, this arrangement proves to be susceptible to pressure fluctuations or surges, since the pressure in one half-cell may significantly exceed the ambient pressure, which is identical to the pressure in the other half-cell. It is therefore necessary after all to design the joints between the half-cells and the diaphragm to be very resistant and consequently to accept the compromises described at the beginning with regard to the measuring accuracy.
It is therefore the object of the present invention to provide a differential pressure sensor which, while of sufficient sensitivity, can also withstand pressure fluctuations.
The object is achieved according to the invention by the differential pressure sensor having: a pressure vessel which is filled with a transfer fluid; a measuring cell which is arranged in the pressure vessel and is surrounded by the transfer fluid, with two half-cells and a diaphragm which separates the two half-cells from each other in a pressuretight manner; two pressure lines which respectively feed the pressure from one of the two measuring points outside the pressure vessel to one of the two half-cells, the pressure supply lines having no fluid communication with the vessel; and at least one pressure accumulator, the pressure of which acts on the transfer fluid in the vessel, and the method comprising the steps of: preparing a differential pressure measuring cell with two half-cells and a separating diaphragm in a pressure vessel; filling the pressure vessel with a transfer fluid; subjecting the transfer fluid to a pressure which is at least as great as the nominal pressure at the measuring points; connecting the two measuring points to a half-cell in each case, of the differential pressure sensor; and compensating for changes in volume of the transfer fluid by means of an elastic pressure accumulator which communicates with the transfer fluid in the pressure vessel.
The transfer fluid in the pressure vessel is preferably subjected to a pressure which is greater than the nominal static pressure of the differential pressure sensor and, as a particular preference, is greater than the maximum values occurring when there are pressure surges.
In a preferred embodiment, the pressure accumulator has at least one elastically deformable body which is suitable for compensating for changes in volume of the transfer fluid caused by temperature fluctuations and for keeping the pressure in the pressure vessel within a predetermined range. Furthermore, the pressure accumulator may comprise a pneumatic component which is compressed when there is increasing volume of the transfer fluid and expands in the converse case. A combination of an elastic body with a pneumatic element is likewise suitable for realizing a pressure accumulator.
In the case of a linearly elastic body, for example a helical spring or some other axially compressible spring in a flexible bellows, the force is proportional to the relative change in length of the elastic body. Consequently, in the case of a pressure accumulator with a linearly elastic body, the increase in pressure is proportional to the relative change in volume of the latter. Therefore, a minimum volume for the pressure accumulator follows from the change in volume of the transfer fluid to be expected over the range of operating temperatures and the permissible tolerance range for the pressure in the pressure vessel.
As an alternative to linearly elastic bodies, the pressure accumulator may also comprise non-linearly elastic bodies with a degressive characteristic. This can be achieved on the one hand by elastic materials with a degressive characteristic or on the other hand by a non-linear articulation of linearly elastic bodies. The non-linearly elastic materials include, for example, shape-memory alloys, for example a work-hardened Nixe2x80x94Ti alloys which, when deflected, form a stress-induced martensite from an austenitic phase. The transition into the martensitic phase produces a degressive characteristic. A non-linear deflection with resultant degressive characteristic can be realized for example by means of a compression spring arrangement with two or more compression springs, the longitudinal axes of which preferably lie symmetrically on the lateral area of a cone with a variable apex angle, the axis of symmetry of which defines the direction of compression of the pressure accumulator. Similar spring arrangements with variable angle of inclination or scroll springs are likewise suitable for achieving a degressive characteristic.
The pressure accumulator may likewise be realized by compressible or elastic packing bodies which are introduced into the pressure vessel. This on the one hand leads to a minimization of the residual volume in the pressure vessel to be filled with the transfer fluid and on the other hand makes it possible for a pressure vessel to be adapted comparatively easily to different sensor elements by filling the intermediate space with packing bodies and transfer fluid in a suitable volume ratio, the transfer fluid being subjected to the required pressure. Examples of suitable packing bodies are solid beads or hollow beads of elastic materials. The use of porous elastic materials likewise allows the porosity of the volume fraction of the transfer fluid in the pressure vessel to be controlled.
Instead of a large number of packing bodies, the interior of the pressure vessel may also be filled with a suitable foam, the proportion of closed pores, open pores and solid material being the parameters by which the volume remaining for the transfer fluid can be controlled.
The method according to the present invention for measuring a pressure difference between two measuring points comprises the steps of preparing a differential pressure measuring cell with two half-cells and a separating diaphragm, in a pressure vessel; filling the pressure vessel with a transfer fluid; subjecting the transfer fluid to a pressure which is at least as great as the nominal pressure at the measuring points; connecting the two measuring points to a half-cell in each case; of the differential pressure sensor; and compensating for changes in volume of the transfer fluid by means of an elastic pressure accumulator which communicates with the transfer fluid in the pressure vessel.